1. Field of the Invention
The present invention relates to a motor driving device in which a AC current supplied from an AC power supply is converted to a DC current which in turn is outputted to a DC link and then converted to an AC current for driving a motor and the AC current is supplied to the motor, and more In particular it pertains to a motor driving device including an initial charging unit that initially charges a smoothing capacitor provided in a DC link.
2. Description of the Related Art
In a motor driving device for driving a motor in a machine tool, a forging press, an injection molding machine, an industrial machine, or various types of robots, AC power inputted from an AC power supply side is converted to DC power and then re-converted to AC power, and the AC power is used as driving power for a motor provided for each drive shaft. Such a motor driving device includes a rectifier that rectifies AC power supplied from an AC power supply side where there is a three-phase AC input power supply and outputs DC power, and an inverter unit that is connected to DC link (direct current link) which is the DC output side of the rectifier and performs mutual power conversion between the DC power of the DC link and the AC power which is driving power or regenerative power of the motor, thereby controlling the speed, torque, or position of the rotor of the motor connected to the AC output side of the inverter. To individually supply driving power to each motor provided respectively in correspondence to a plurality of drive shafts, thereby driving and controlling the motors, the inverter unit is provided by a number equal to the number of the motors. The rectifier is usually provided one for the plurality of inverter units for the purpose of reducing the cost and occupancy space of the motor driving device.
For example, as described in Japanese Unexamined Patent Publication No. H06-311639, a smoothing capacitor (referred to also as “DC link capacitor”) is provided on the DC link side of the inverter unit to which the rectifier is connected. The smoothing capacitor has the function of restraining a pulsating component of the DC output of a converter and is capable of accumulating DC power. Since it is preferable that the smoothing capacitor be subjected to initial charging (referred to also as “preliminary charging”) between immediately after the motor driving device is started and before driving of the motor is started (i.e., before power conversion operation by the inverter unit is started), it is common that an initial charging unit therefor is provided.
FIG. 8 is a view illustrating a configuration of a general motor driving device. The motor driving device 1000 includes a rectifier 111 that rectifies AC power from a commercial three-phase AC power supply 3 and outputs DC power, and an inverter unit 112 that is connected to a DC link, which is the DC side of the rectifier 111, and converts the DC power outputted from the rectifier 111 to AC power of a desired voltage and a desired frequency supplied as driving power of a motor 2 or converts AC power regenerated from the motor 2 to DC power, thereby controlling a speed, a torque, or a position of a rotor of the motor 2 connected to the AC side of the inverter unit 112.
To individually supply driving power to each of motors 2 provided respectively in correspondence to a plurality of drive shafts, thereby driving and controlling the motors 2, the inverter unit 112 is provided by a number equal to the number of the motors 2 and connected in parallel to each other. A smoothing capacitor 221 is provided on the DC input side of each inverter unit 112. In other words, the smoothing capacitor 221 is located on the DC link side of the inverter unit 112 to which the rectifier 111 is connected. In FIG. 8, the number of the motors 2 is three by way of example, and hence the number of the inverter units 112 is three. The rectifier 111 is provided one for the plurality of inverter units 112 for the purpose of reducing the cost and the occupancy space of the motor driving device 1000.
It is preferable that the smoothing capacitor 221 be subjected to initial charging between immediately after the motor driving device 1000 is started (i.e., immediately after the opening/closing unit 116 is closed (on)) and before driving of the motor 2 is started (i.e., before power conversion operation by the inverter unit 112 is started). A large inrush current flows through the rectifier 111 between a state in which no energy is accumulated in the smoothing capacitor 221 and immediately after initial charging is started. In particular, the larger the electrostatic capacitance of the smoothing capacitor 221, the larger inrush current occurs. To countermeasure such inrush current, the motor driving device 1000 is commonly provided with an initial charging unit 113 between the rectifier 111 and the smoothing capacitor 221 in the inverter unit 112. For example, as illustrated in FIG. 8, when a plurality of the inverter units 112 are connected in parallel to each other, the smoothing capacitors 221 are also connected in parallel to each other correspondingly, but the initial charging unit 113 is provided one between the rectifier 111 and the smoothing capacitors 221.
The initial charging unit 113 includes a switch 222 and a charging resistor 223 connected in parallel to the switch 222. The switch 222 is opened (off) only during the initial charging period of the smoothing capacitor 221 immediately after the motor driving device 1000 is started, and maintains a state in which it is closed (on) during the normal operating period for which the motor driving device 1000 drives the motor 2. An example of the switch 222 includes, for example, a thyristor. More specifically, during the initial charging period between immediately after the motor driving device 1000 is started and before driving of the motor 2 is started, upon the switch 222 being opened (off), a DC current outputted from the rectifier 111 flows in the smoothing capacitor 221 through the charging resistor 223, and thus the smoothing capacitor 221 is charged. When the smoothing capacitor 221 is charged to a predetermined voltage, the switch 222 is closed (on), and thus the opposite ends of the charging resistor 223 are short-circuited, and thus the initial charging operation is completed. Thereafter, the inverter unit 112 starts a power conversion operation and supplies driving power to the motor 2, and the motor 2 is driven based on the driving power.
As described above, during the initial charging period of the smoothing capacitor 221, upon the switch 222 being opened (off), the DC power outputted from the rectifier 111 flows through the charging resistor 223 and is consumed as heat in the charging resistor 223, and thus occurrence of an excessive inrush current during the initial charging period is restrained. However, the charging resistor 223 has instantaneous load capability (hereinafter, referred to simply as “load capability”) defined as an amount of heat that it can withstand being fused off, and hence when an amount of heat generated due to a current flowing through the charging resistor 223 exceeds the load capability, the charging resistor 223 will be fused off.
Since the DC current that flows through the charging resistor 223 during the initial charging period depends on the electrostatic capacitance of the smoothing capacitor 221 and the amount of heat generated in the charging resistor 223 depends on the DC current flowing through the charging resistor 223, the load capability of the charging resistor 223 that can be provided in the motor driving device 1000 is determined depending on a maximum electrostatic capacitance of the smoothing capacitor 221. For example, in a system that the electrostatic capacitance of the smoothing capacitor 221 is large, it is preferable to use a resistor having a large rated power as the charging resistor 223. As illustrated in FIG. 8, when a plurality of the inverter units 112 are connected in parallel to each other, a plurality of the smoothing capacitors 221 are also correspondingly in a parallel-connected relationship with each other, and hence, in such an instance, the load capability of the charging resistor 223 is determined depending on a combined capacitance of the electrostatic capacitances of all the smoothing capacitors 221. Therefore, when designing the motor driving device, the designer usually selects the charging resistor that can be provided in the motor driving device, based on the maximum electrostatic capacitance of the smoothing capacitor and taking into account of the relationship between the load capability of the charging resistor and the maximum capacitance of the smoothing capacitor that can be provided under the load capability.
Even in a case in which the charging resistor that can be provided in the motor driving device is selected based on the electrostatic capacitance of the smoothing capacitor, when the time taken for initial charging of the smoothing capacitor is longer for some reason, there is a possibility that since an amount of heat generated due to a current flowing through the charging resistor exceeds the load capability, the charging resistor will be fused off. In particular, when the plurality of inverter units are connected in parallel to each other, the plurality of smoothing capacitors are also connected in parallel to each other, and in such a case, there is a tendency that the time taken for initial charging of the smoothing capacitors is longer, and thus there is a high possibility that the charging resistor will be fused off.
Further, as described above, when designing the motor driving device, the designer selects the charging resistor that can be provided in the motor driving device, based on the maximum electrostatic capacitance of the smoothing capacitor. Thus, when the maximum electrostatic capacitance of the smoothing capacitor is large, such as for example when a plurality of the inverter units are connected in parallel to each other and a plurality of the smoothing capacitors are also connected in parallel to each other, it is preferable to select a charging resistor having high load capability, and thus there is a problem in which the cost of the motor driving device is increased and the motor driving device is large-sized.
Further, when a substitution is made with inverter units each including a smoothing capacitor having a higher electrostatic capacitance or an additional inverter unit or inverter units are added by reason of later maintenance or design change, it is likely that a combined capacitance of the electrostatic capacitances of the plurality of smoothing capacitors exceeds the maximum capacitance of the smoothing capacitor which is determined based on the load capability of the charging resistor which is already in use. In this case as well, the time taken for initial charging of the smoothing capacitor is longer, and there is a possibility that since the amount of heat generated due to a current flowing through the charging resistor exceeds the load capability, the charging resistor will be fused off.